how do I modulate a key in the middle of the song?

A GB project can only have one key. But nobody keeps you from playing in a different key than the one your project is in. (To answer more specifically, we'd have to know what you are doing and what you want to do: recording real or software instruments, manipulate existing recordings, working with loops ...)

Brian, are you asking about GarageBand on your iPad or in your Mac? On your Mac you can transpose parts of your song using the automation curves in the Master Track or edit single notes in the Track Editor, but on the iPad you simply have to play the notes the way you want them to sound. You cannot edit single notes, only trim and move regions.

GarageBand for iPad: GarageBand